The invention pertains to a low-pressure hoisting bolster or air cushion with an airtight chamber the wall of which consists of a fabric covered with rubber or synthetic material and is fitted with a valve having a nozzle which consists of a valve-socket inserted through a hole in the wall and a ring-fastener attached to it and to the inner side of the wall.
Low-pressure hoisting air cushions of the type mentioned initially are distinguished from high-pressure air cushions by the fact that the wall fabric is elastic. In high-pressure cushions, a cord or similar type of material capable of taking great pressure (which may be a type of steel or synthetic material) is incorporated into the substantially thicker walls. This cord so reinforces the walls of the cushion that expansion in the direction of the reinforcing is practically excluded. Low-pressure hoisting air cushions are, for example, manufactured in the range of 5 bar; whereas high-pressure air cushions can accommodate much higher pressures.
Low-pressure air cushions are known for example from English Patents 1,355,315 and 1,405,888. Both publications show cylinder-shaped hoisting cushions with a valve fitted in the cylinder jacket. This valve consists of a nozzle which is introduced through a hole in the cylinder jacket and which with its ring-fastener lies flat on the inside wall. The nozzle is made of rubber and its tube-shaped valve socket merges in one piece into the ring-fastener. The ring-fastener in particular has a wall strength considerably greater than the wall into which the nozzle is glued. In this way the material of the wall in the vicinity of the ring-fastener is appreciably strengthened with the result that the elasticity of the material is considerably reduced. If one moves from this area (which is thickened by the ring-fastener) to the outside the elasticity of the material of which the wall is composed abruptly sinks at the outer edge of the ring-fastener to an appreciably lower value, namely that of the normal material without reinforcing. In this area and somewhat beyond it there appear when the air cushion is inflated small creases which spread out radially and in essence radiate out from the central point of the hole in the wall. During inflation the wall-fabric of course stretches, but because the elasticity is greater beyond the reinforcing ring-fastener than in the neighborhood of the ring-fastener there inevitably occur at the transition points mentioned irregularities which manifest themselves in the small creases (they are referred to below as microcreases) to which attention has already been drawn. These place considerable stress on the cohesion between the ring-fastener and the wall fabric, and spot weaknesses occur in this adhesion, and loosening, and partial stripping. Thus leaks originate between the nozzle and the coated material which surrounds the hole in the wall. The air cushion becomes unserviceable and must, if it is possible, be repaired.
The reinforced area between the nozzle and the wall, more accurately that part between the ring-fastener of the nozzle and the coated material which encompasses the weak area, is the punch position of the familiar low-pressure air cushion. In consequence, the explosion limit (bursting point) of the existing cushions is for all practical purposes determined exclusively by the quality of this area of junction. But concerning the existing cushions their behavior over a long period of time is also influenced primarily by this area of junction because they too in every normal process of inflation completed with pressures well under the explosion limit form microcreases which in the course of time enlarge and cause leakages.
Taking these observations as its starting point the invention set out to avoid the disadvantages of the existing low-pressure hoisting air cushions noted initially and to develop the cushion further in such a manner that in spite of the implementation of elastic material for the wall no uneven areas, microcreases or the like should occur during inflation between the nozzle and the surrounding wall area so that the weakening of the uniformly consistent wall in the region of the hole does not result in a mechanical weakening of the whole cushion.
Proceeding from the low-pressure hoisting air cushion of the kind noted initially this task was solved by including between the ring-fastener and the wall a compensation ring which has a greater diameter than the ring-fastener and is preferably composed of a material which has a greater elasticity than the ring fastener and the material of the wall and which on one side is so attached to the ring-fastener and on the other side to the wall via a circular joining surface that between the inner edge of this circular joining surface and the outer edge of the ring-fastener there is preserved a circular compensatory zone over which the compensation ring is linked to neither the wall nor the nozzle.
The distinguishing factor of this low-pressure hoisting air cushion is the fact that the ring-fastener of the nozzle is not linked directly but rather by the insertion of the compensatory ring to the wall. In this way the compensating ring is linked on both sides (on the one side with the ring-fastener of the nozzle and on the other with the material of the wall) in such a way that there is created between both areas where it is joined/attached a circulatory compensatory zone over which the compensatory ring remains free on both its sides. That is, it is not linked to any other part. This method of construction ensures that the nozzle can be moved backwards and forwards relative to the wall. In the low-pressure hoisting air cushions described initially such independent motion of the nozzle was impossible as it was linked directly to the wall fabric. When in use the relative moveability of the nozzle relative to the wall is not a negative factor because by reason of the internal pressure the ring-fastener is pressed against the inner side of the wall. In order to avoid the possibility that when the cushion is inflated the nozzle through ignorance or clumsiness is pressed too forcefully into the inner area (which would result in considerable overloading of the circular joining surface between the compensation ring and the wall) it is proposed in a future development so to reinforce the nozzle outside/beyond the hole that it can be pushed only marginally into the inner area of the cushion.
By reason of the compensation ring there results a carefully graduated transition to the hole in the wall and to the surrounding wall area on the one hand and on the other to the wall and to the nozzle. Because the hole with its inner edges is no longer linked directly to the nozzle but can be moved opposite it the weaknesses in the uninterrupted character of the wall area and the linkage between jacket and nozzle can result independently of each other and be dealt with most expeditely separately from each other. Thus in a further development of the invention the region around the hole can be reinforced by at least one reinforcing ring thereby preventing the development of tears at the edge of the hole. With respect to the nozzle the relatively stiff material of which it is made (generally rubber which is several millimeters thick) is no longer joined directly to the wall fabric; rather compensation is created via the compensation ring. As the material of the compensation ring has a greater elasticity than the material of the wall the relatively stiff nozzle influences only minimally the expansion process of the wall during inflation. With respect to the nozzle the uninterrupted, even quality of the wall is impaired only by the fact that the wall is thickened in the vicinity of the circular joining surface by the extra material of the compensation ring. On the other hand, this thickening protects the inner hole and can be so calibrated with the reinforcing rings mentioned that optimal conditions during inflation are created. In other words the fact that a hole must of necessity be fitted into the wall need not noticeably impair the mechanical expansion characteristics during inflation.
The advantage of the compensation ring pertinent to the invention is to be sought in the fact that production of the low-pressure hoisting air cushion remains simple and the area where nozzle and wall join is only minimally thickened it remains for all practical purposes as flat as in the existing low-pressure hoisting air cushions.
In its simplest form the compensation ring is a simple washer which is attached (preferably vulcanized) to the ring-fastener of the nozzle. The compensation zone is situated concentrically outside the described joining area while the circular joining area is outside of the compensation zone. The compensation ring can also be incorporated as a bellows. In its simplest form, it is composed of two washers similar to the one described initially. The lower washer is as described above joined to the ring-fastener of the nozzle (ideally by vulcanization) and is free at the adjoining ring area and joined to the second (upper) washer at its outer edge. This second washer is joined to the material surrounding the hole in the wall at the region of the inner ring.
The last-mentioned solution is more complicated and not as cheap as the solution with just one washer as compensation ring but it does have the advantage that the total diameter can be smaller and the join be effected directly in the vicinity of the hole.